Precharging device for electrostatic painting apparatus



w. s. PETTIGREW ETAL 3,219,013 PRECHARGING DEVICE FOR ELECTROSTATIC PAINTING APPARATUS Nqv. 23, 1965 Filed May 4, 1962 POWER SUPPLY SOURCE POWER SUPPLY m @5a W Wndw/n. wlw

m W m N United States Patent() 3,219,013 PRECHARGING DEVICE FOR ELECTROSTATIC PAINTING APPARATUS William S. Pettigrew, Pleasant Ridge, and Warren D. Hill,

Royal ak, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 4, 1962, Ser. No. 192,404 2 Claims. (Cl. 118-627) This invention relates to apparatus for electrostatic spray painting.

It has been previously known to apply paint to an article by applying paint to a rotating disc and centrifugally atomizing the paint from the edge of the disc so that it is thrown in the general direction of the article and to deposit the paint on the article by electrically charging the paint particles and providing an electrical field for attracting the charged particles to the article. One difiiculty with this type of arrangement is that, if the electrical field extends from the edge of the rotating disc to the article, then the paint tends to disperse into a wide band whereas it is often desirable to have the paint applied to the article in a narrow band. When the paint is dispersed into a wide band the paint particles at the edge of the band are often deposited on the article in a dry or semi-dry condition and hence cannot flow together to produce a smooth film on the article. This results in a rough or orange peel surface. The drying of the particles at the edge of the band is believed to be caused by at least the following conditions: (l) those particles have a longer flight path to reach the workpiece than do those in the center of the band, hence permitting excessive evaporation of solvent, and (2) when the particles are well dispersed there is a low concentration of solvent vapor around the particles hence permitting rapid solvent evaporation whereas when the particles are confined to a small band, the concentration of solvent vapor is greater and the rate of evaporation is lower.

It is the goal of the present invention to provide an improved electrostatic painting apparatus wherein the width of the band of paint applied to the article can be controlled. The invention is carried out by providing a rotating disc, field electrodes above and below the disc, and means for charging the paint before it is applied to the disc.

The above and other advantages of the invention will be made more apparent from the following specification taken with the accompanying drawings wherein like reference numerals refer to like parts and wherein:

FIGURE 1 is a schematic elevational View of a painting apparatus according to the invention;

FIGURE 2 is a detailed cross-sectional view of the painting head of FIGURE 1; and

FIGURE 3 is a cross-sectional view of a precharging means according to the invention.

When coating material is atomized from the edge of a rotating disc the particles tend to disperse into a wide band. When the particles are electrically charged and an electrostatic field is established between the disc edge and an article spaced therefrom, the particles are even further dispersed because the field diverges from the disc edge. However, by establishing an electrostatic field which converges from the `disc edge, the charged particles will be compressed into a relatively narrow band. The field electrodes described herein help establish such a convergent field.

FIGURE 1 illustrates a painting head 10 suspended from a support member 12 by a hollow insulating column 14. An electrical motor 16 is mounted above support member 12 and is connected in driving relationship to a paint disc 18 by a drive shaft 20 extending through the insulating column 14. Preferably the disc 18 is made of 3,219,013 Patented Nov. 23, 1965 ICC an insulating material such as phenolic resin, although a metal disc may be used provided that it is carefully insulated from other portions of the head 10. The disc 18 contains a central cup portion 22 which receives paint from a feed tube 24 and aids in distributing the paint in a thin film over the upper surfaces of the disc 18 as it rotates. In addition to the disc 18, the painting head 10 comprises an upper metallic field electrode 26 secured to the insulating column 14 and a lower field electrode 28 supported from the upper electrode 26 and electrically connected thereto by a plurality of bolts 30. Insulating spacers 32 around the bolts maintain the desired distance between electrodes 26 and 28. Preferably the electrodes 26 and 28 are covered by some insulating material 31 such as Varnish or Tefion so that no metallic portions of the electrode are exposed, although in many cases it is sufficient to insulate only the surfaces of the electrodes facing the disc. The paint feed line 24 is composed of an insulating material such as nylon tubing and extends from the cup 22 of disc 18 to a paint source which meters a predetermined paint ow to the disc.

A charging device 34 comprises a conventional brass hose coupling 36 in the feed line 24 and has a sharpened needle 38 within the feed line connected to the coupling 36. As illustrated, a screw 40 is threaded through a tapped hole in the side of the coupling 36 and held in place by a lock nut 42. The needle 38 is then soldered to the end of the screw 40. The needle 38 is long enough to extend a few inches beyond the coupling 36. One side of a power supply 44 is connected to the charging device 34 while the other side is connected to a metallic collar 46 surrounding the paint feed line 24 so as to be insulated from the paint flowing through the line 24 and axially spaced from the charging device 34. The power supply 44 preferably has an output of 50,000 to 100,000 volts although the voltage must not be so great as to cause arcing between coupling 36 and collar 46.

An article 50 to be painted is suspended by a conductive hanger 52 from an overhead conveyor 54. If desired, the hanger 52 may be covered with an insulating material, not shown. The overhead conveyor 54 is preferably a loop conveyor which forms at least half a circle having the painting head 10 as its center so that paint radiating from the head in all directions may be utilized. A second power supply 56 has one side connected to the field electrode 26 and its other side connected to the con-v veyor 54. In practice, the conveyorside of the power supply is usually electrically grounded. The article 50 is at the same potetntial as the conveyor 54 due to its connection thereto by the conductive hanger 52. The power supplies 44 and 56 are so connected that the charging device 34 and the field electrodes 26 and 28 will be of the same polarity; for example, they may be negative, while the positive side of the power supplies are connected to the collar 46 and the conveyor `54, each of which is illustrated as being connected to ground. The power supply 56 is usually selected to provide about 100,000 volts between the field electrodes 26 and 28 and the article 50, thus providing an electrostatic field originating on electrodes 26 and 28 which converges toward its terminal on article 50. Even if the disc 18 should also be charged to 100,000 volts, the shape of the field would not be appreciably altered since the field electrodes are somewhat closer to the article 50 than the disc 18. The field electrodes serve as elecrostatic shields and the field intensity between the shields near the edge of the disc 18 would be very low and may even approach zero depending upon the geometry of the painting head 10. It has been found that a satisfactory arrangement comprises a disc 18 twenty-four inches in diameter, field electrodes 26 and 28 twenty-eight inches in diameter each spaced Vertically two inches from the -disc 18, and a.distance of 3 twenty-four inches between the articles 50 and the edge of disc 18.

Heretofore the drawback of using field electrodes of the type described to control the shape of the electrostatic field between the painting head and the article is that, since the electrostatic field near the disc 18 is very low, there is no means to charge the paint either before or during its atomization. Unless the paint becomes charged the electrostatic field between the painting head and the article cannot assist in the deposition of paint on the article. There have been attempts to charge the paint particles after atomization but these have had only limited success. According to this invention, however, the precharging device 34, 46 will charge the paint in the feed line 24 and the charge will remain on the paint until it has been atomized and deposited on the workpiece. An electrostatic field exists -between the needle 38 and the collar 46 spaced therefrom and since the end of needle 38 is sharpened, the field at that point'will be very intense thereby providing an efiicient means of charging the paint flowing through the feed line 24 past the point of needle 38. The electrons injected into the paint in the feed line flow with the paint to the disc and are carried by the atomized particles to the workpiece 50. The electrons are prevented from leaking through the air from disc 18 to either of the field electrodes by the coating of insulation 31 on those electrodes. In some instances it is permissible to omit the collar 46 if the voltage drop in the feed line due to the current therein is sufficient to provide an intense field at the tip of needle 38. In such instances there must be several thousand volts differential between the disc 18 and needle 38. The charging device 34 and collar 46 must be carefully located so that arcing does not occur to a point of different potential; for example, between the grounded collar 46 and the field electrode 26.

In summary, it will be seen that as paint passes through the feed line 24 it receives a charge due to the intense field at the point of needle 38. The charged paint passes to the disc 18 and is centrifugally atomized by the rotation thereof and is thrown outwardly generally toward the articles 50 surrounding the painting head 10. As the charged particles move .beyond the field electrodes 26 and 28 they are acted upon by that field. Due to the convergent shape of the field the paint particles tend to disperse less and produce a narrower band than if the field had originated at the disc 18. The band width of the paint spray pattern can be controlled by selecting appro- 5() priate field electrode spacing and diameter.

The embodiment of the invention described herein is for illustrative purposes only and the scope of the invention is t0 be limited only by the following claims:

1. Electrostatic painting apparatus for painting an article spaced therefrom comprising a centrifugal atomizing means, a feed line of insulating material for supplying paint thereto, charging means connected to the feed line for electrically charging the paint, said charging means comprising a conductor having a sharp edge disposed within the feed line and a second conductor spaced from the sharp edge disposed outside surrounding the feed line and means for establishing a potential difference between the said conductors and for establishing an intense electrostatic field adjacent the sharp edge, field electrodes surrounding the atomizing means insulating material covering the field electrodes, and means for providing an electrostatic field between the field electrodes and the article.

2. Electrostatic painting apparatus for painting an article spaced therefrom comprising a centrifugal atomizing means, a feed line of insulating material for supplying paint thereto, charging means connected to the feed line for electrically charging the paint, said charging means comprising a conductor having a sharp edge disposed within the feed line and a second conductor spaced from the sharp edge surrounding the feed line and means for establishing a potential difference between the said conductors and for establishing an intense electrostatic field adjacent the sharp edge, field electrodes surrounding the atomizing means, means for preventing current leakage between the atomizer and the field electrodes, and means for providing an electrostatic field between the field electrodes and the article.

References Cited by the Examiner UNITED STATES PATENTS 2,509,277 5/1950 Ransburg et al. ll7-93.4 2,827,394 3/1958 Sherman 118-626 2,899,136 `8/1959 Reindl 118-621 2,955,565 10/'1960 Schotland 1118-627 X 3,031,145 4/ 1962 Croskey 118-627 3,057,558 10/1962 Verba 118-627 3,063,642 11/1962 Point 118-627 3,104,185 9/1963 Reindl 118-627 3,147,145 `9/*1964 Simmons 118-624 FOREIGN PATENTS 556,965 5/ 1958 Canada. 616,933 3/1961 Canada.

CHARLES A. WILLMUTH, Primary Examiner.

RICHARD D. NERVIUS, Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,219,013 November 23, 1965 William S. Pettigrew et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 10, strike out "disposed outside".

Signed and sealed this 20th day of September 1966.

(SEAL) Att/est:

ERNEST W. SWIDER Attestng Officer EDWARD J. BRENNER ioner of Patents 

1. ELECTROSTATIC PAINTING APPARATUS FOR PAINTING AN ARTICLE SPACED THEREFROM COMPRISING A CENTRIFUGAL ATOMIZING MEANS, A FEED LINE OF INSULATING MATERIAL FOR SUPPLYING PAINT THERETO, CHARGING MEANS CONNECTED TO THE FEED LINE FOR ELECTRICALLY CHARGING THE PAINT, SAID CHARGING MEANS COMPRISING A CONDUCTOR HAVING A SHARP EDGE DISPOSED WITHIN THE FEED LINE AND A SECOND CONDUCTOR SPACED FROM THE SHARP EDGE DISPOSED OUTSIDE SURROUNDING THE FEED LINE AND MEANS FOR ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN THE SAID CONDUCTORS AND FOR ESTABLISHING AN INTENSE ELECTROSTATIC FIELD ADJACENT THE SHARP EDGE, FIELD ELECTRODES SURROUNDING THE ATOMIZING MEANS INSULATING MATERIAL COVERING THE FIELD ELECTRODES, AND MEANS FOR PROVIDING AN ELECTROSTATIC FIELD BETWEEN THE FIELD ELECTRODES AND THE ARTICLE. 